Convert AppSettings to Roslyn Generator

[dansiegel]

Description

Updates the AppSettings to utilize a Roslyn Generator rather than MSBuild Task. This provides some additional power in that you will now be able to declare a partial class with an implemented interface and have the Mobile.BuildTools generate the missing properties for you.

public interface IB2COptions
{
    string ClientId { get; }
    string Domain { get; }
}

public partial class B2COptions : IB2COptions
{
    // properties generated by Mobile.BuildTools
}

Then in the buildtools.json you might have:

{
  "appSettings": {
    "MyProject": [
      {
        "className": "B2COptions",
        "properties": [
          {
            // Declare the properties
          }
        ]
      }
    ]
  }
}